Coal seams contain significant quantities of natural gas. This natural gas is composed primarily of methane. The rate of recovery of methane from coal seams typically depends on the rate at which gas can flow through the coal seam to a production well. The gas flow rate through a coal seam is affected by many factors including the matrix porosity of the coal, the permeability of the coal seam, the extent of the fracture system which exists within the coal seam, and the stress within the coal seam.
An unstimulated coal seam has a natural system of fractures, the smaller and most common ones being referred to as "cleats" or collectively as a "cleat system". To reach a wellbore, the methane must desorb from a sorption site on or within the coal matrix and diffuse to the cleat system. The methane travels along the cleat system and other fractures present within the coal seam to the wellbore where it is recovered.
Typically, the natural system of fractures within a coal seam does not provide for an acceptable methane recovery rate. In general, coal seams must be stimulated to enhance the recovery of methane from the seams. Typically, the stimulation is completed prior to placing a production well on-line to a gas gathering system.
Various techniques have been developed to stimulate coal seams. One example of a technique for stimulating the production of methane from a coal seam is to complete the production wellbore with an open-hole cavity. In this technique, a wellbore is drilled to a location above the coal seam to be stimulated. The wellbore is cased and the casing is cemented in place using a conventional drilling rig. A modified drilling rig is then used to drill an "open-hole" interval within the coal seam. An open-hole interval is an interval within the coal seam which has no casing set.
The open-hole interval can be completed by various methods. One method utilizes injection/blowdown cycles to create a cavity within the open-hole interval. In this method, air is injected into the open-hole interval and then released rapidly through a surface valve. Once a suitable cavity has been created, the modified drilling rig is removed from the wellbore and the production well is put into service. A metal liner, which has holes, may be placed in the open-hole interval if desired. The coal seam will be dewatered if necessary to improve the desorption of methane from the coal seam.
Generally, once a coal seam has been dewatered and a sufficient methane recovery rate is maintained from the production well, very little is done to the production wells or the coal seam other than to perform routine and preventative maintenance on the production equipment.
As used herein, the following terms shall have the following meanings:
(a) "coal seams" are carbonaceous formation which typically contain between 50 and 100 percent organic material by weight;
(b) "cleats" or "cleat system" is the natural system of fractures within a coal seam;
(c) "formation parting pressure" and "parting pressure" mean the pressure needed to open a coal seam and propagate an induced fracture through the coal seam;
(d) "reservoir pressure" means the pressure of a coal seam near a well during shut-in of that well;
(e) "recovery" means a controlled collection and/or disposition of a gas, such as storing the gas in a tank or distributing the gas through a pipeline. "Recovering" specifically excludes venting the gas into the atmosphere;
(f) "sorption" refers to a process by which a gas is held by a carbonaceous material, such as coal, which contains micropores. The gas typically is held on the coal in a condensed or liquid-like phase within the micropores, or the gas may be chemically bound to the coal;
(g) "original methane-in-place" means the quantity of methane sorbed to the carbonaceous material of the coal seam available to be drained by a wellbore penetrating the seam. The original methane-in-place is measured prior to the initial recovery of methane from the coal seam; and
(h) "pore pressure cracking" is shear failure which is induced in weak formation, such as coal seams, by rapidly changing the pressure which is present within the micropores and the macropores of the carbonaceous matrix of the coal seam. Such failure will usually be accompanied by an increase in permeability of the coal seam.